Dust filter



Jang, 249 W5@ H. J. HERSEY, .JR 2,495,635

DUST FILTER Original Filed Feb. 9, 1944 3 Sheets-Sheet l jam. 24 E5@ H. J. HERSEY, .JR

DUST FILTER :s shets-sheet 2 Original Filed Feb. 9, 1944 HV1/EN TOR Hen/'7g I 6fm-:my 71? y La @fuffwg wam m m5@ H, l HERSEY, JR 2,495,635

DUST FILTER Original F'iled Feb. 9, 1944 3 Sheets-Sheet 3 Patented jan. 24, 1930 `1NITED STATES OFFICE February 9, 1944.

1946, Serial No. 683.018

(Cl. 18S-61) 3 Claims. l

This invention relates to a method and apn paratus for filtering dust-laden gases of the type in which the lter medium is cleaned without interrupting the filtering operation by a jet of re1- atively high-pressure gas blown through the filter medium in the reverse direction to the flow of gas being filtered.

This application is a continuation of my application Serial Number 521,695, filed February 9, 1944, now abandoned.

More particularly the invention relates to the ltering of air or other gases containing particles of very small size.

For the filtering of fine dusts ordinary woven fabric is not satisfactory. Unless the threads are so ne that the fabric is too delicate for extended commercial use, the interstices are too large to block the passage of the smaller particles. To give the desired mechanical strength coupled with high filtering power as regards the maximum size particle which can pass, it is necessary to use a fabric having interstices much smaller than its thickness. Felt has such characteristics.

Felt has been proposed for use as a filter medium and has been found to be highly etcient when first used. The trouble has been that the pores of the felt soon clog up so that the amount of air passing falls oii to a point at which the capacity of the filter is so low as to be impracticable.

It has been proposed to clean screens and filters of woven fabric by projecting a stream of air from a slotted tube against the surface opposite to that on which the dust collects toy remove the dust particles caught in the meshes of the fabric. However, this reverse flow jet cleaning system does not appear to have been applied to close dense thick filter elements of the felt type.

I have found that certain conditions should be observed to obtain good results by thls'reverseflow jet cleaning method as applied to filter mediums of the felt type.

With woven fabric it is often suiiicient to direct the jet of cleaning air towards the outer surface of the fabric, the inertia of the cleaning air being sufficient to carry it through the fabric pores. In other Words it is not necessary with such material, to have the mouth of the cleaning air iet in contact with the fabric.

However, with a filter medium having inter stices much smaller than its thickness, such as felt, the resistance to the iiow of air is so great that the inertia of the cleaning air is not sufficient to carry it through the filter medium. The tendency is for the air to escape laterally instead of passing through the lter.

' This means iirst, that the filter medium must be iiexlble or the jet must be resiliently mounted This application July 12,

2 so that as the jet moves to and from over the lter surface the two can remain in contact.

Second, as the pressure of the cleaning air tends to push the filter surface away from it there must be a force provided acting in opposition to and greater than the force produced by the cleaning air.

When these two conditions are complied with the lateral escape of cleaning air is largely if not entirely prevented so that substantially all of the latter is compelled to flow through the felt.

The precise mode by which a filter medium of the felt type retains the particles suspended in air is uncertain. This uncertainty is due largely to the fact that particles much smaller than the passageways through the felt are retained. Conseqeuently, there seems to be more than a true screening-out action, such as exists in the case of a woven-wire screen. Apparently as the particles pass through the tortuous passages they are thrown against and adhere to the passage walls. Further particles contact these adhering particles and the process is frequently continued until the passageways are bridged-over. This action is probably confined chiefly to the layers of the felt medium next to the high-pressure side of the filter.

Simultaneously with and after the deposition of particles in the surface layers of the felt, there is also a building up of a dust layer on top of the felt surface. rThis dust layer, if undisturbed, increases in thickness and density to the point at which the filter ceases to act eiiiciently.

The cleaning air, therefore, does two things. First, it keeps open a suicient number of the passageways through the felt to maintain the desired porosity. Second, it breaks off the dust layer on top of the felt or at least opens enough channels therethrough to permit air to flow into the felt beneath.

The cleaning of the filter medium by reversing the direction of iiov/ of air therethrough involves moving these adhering particles and masses of particles in the opposite direction to which they entered the interstices. Mere reversal of air flow does not insure that each particle is carried back along the same passageway by which it entered until it finally leaves the filter medium. Particles become wedged in between the fibers or enter pockets through which the cleaning air might not flow or through which it might flow with suflicient velocity to dislodge them.

Two other conditions are, therefore, required for sufficient cleaning of the filter. First, the velocity of the cleaning air through the filter should be greater than the velocity of the air being filtered. In that way a dislodging force is obtained which is greater than the force which caused the particles to be retained in the interstices or the filter during ltering.

Secondly, that portion of filter medium in register with the cleaning jet should be stretched on the side on which the air to be ltered enters and on which the cleaning air leaves the lter medium. This stretching elongates the interstices in the direction in which the stretching force is exerted. This elongation is probably accompanied by a contraction of the interstices in a direction perpendicular to that in which the elongation occurs. Whether the contraction is greater or less than the elongation does not appear to affect the cleaning. In either event, there is a distortion of the passageways which tends to break up any particle bridges which formed during the ltering part of the cycle and makes easier the removal of their component particles. Further, any relative movement of the fibers composing the lter medium helps to dislodge any particle wedged in between two or more ibers.

Examination of sections of felt which have been in use for several months shows that practically all the dust is caught in the first half of the felt layer. Consequently it is suicient for cleaning purposes to stretch the inside half only. The outer half may be compressed somewhat without affecting the liberation of trapped particles.

If, therefore, the felt while under tension as a whole is bent around a slotted pipe supplied with the cleaning air, that part of the felt in contact with the pipe will be under compression while the other parts will be under tension.

These various conditions may be most readily complied with by using as the iiltering medium a felt cylinder into the upper part of which the dust-laden air is introduced while the separated dust is collected and drawn off from the bottom. Surrounding the felt cylinder is a tubular ring supplied with relatively high pressure air from an air compressor. This ring as a whole has an internal diameter somewhat less than the external diameter of the cylinder when the latter is distended by the pressure of the air to be ltered. Around the inner periphery of this ring is a slot for the discharge of the cleaning air.

As the ring is smaller than the felt cylinder the latter curves around the inner part of the ring, and, due to the pressure inside the cylinder, the inner1 part of the felt at this point is under tension and its interstices are thereby expanded. The mutual action of the ring and cylinder as well as the relationship between the forces set up by the pressure of the air to be filtered and the pressure of the cleaning air will be set forth inY detail in connection with the description of the specific form of construction shown in the drawings.

The reverse iiow cleaning jet merely removes the dust particles from the interstices in the lter and from the inner wall of the bag and projects them back into the interior of the latter from whence they came. The cleaning air so introduced then has to flow outwards through the filter along with the air to be ltered. Unavoidably some of the dust removed from the nlter is re-deposited in and on the ilter. To get efficient operation the percentage re-deposited should be as small as possible and further its point of re,-` deposit should be as far below the point of initial deposit as practicable.

No progress would be made if all the dust removed by the cleaning jet came backonto, thev same or a higher section of the iilter medium. large part of the dust so removed must either drop by gravity or be carried by air currents to the collecting space at the bottom of the lter chamber or be re-deposited on a lower section of the nlter medium.

It appears that to reduce the percentage of redeposit as far as possible, the dust removed should be carried to the central part of the air space inside the lter bag where some of it can coagulate and drop by gravity. Further, as the dust is some distance from the iilter wall its re-deposit in part at least is delayed. If the dust removed is almost immediately carried back onto and into the filter little progress will be made.

These conclusions are supported by actual observations of the change in pressure inside the lter bag as the cleaning ring reciprocates upand down. Especially with tall lter bags there is a markedly greater drop in pressure inside thel bag on the down stroke of the cleaning ring than on the upstrole. The actual cleaning action must be approximately the same on both strokes so that the difference in pressure drops would appear to be due to a difference in the rate at which the clean sections become clogged again.

The general flow of air is from the top down. Hence, if the clean section of the bag is above the cleaning ring as it is when the ring is moving downwards, the rush of air to the clean section will not go past the section being cleaned. That means that such rush of air does not prevent the cleaning air carrying the dust it has removed to the central portions of the filter` bag. If, on thev other hand, the clean section of the bag is below the cleaning ring (as it is when the ring, is mov.- ing upwards), the downward rush of air to the clean section will sweep by the section being cleaned and carry with it the dislodged dust and sweep it back into and onto the filter.

As the volume of cleaningair should be kept as low as possible, the cleaning air should.l be used only when it can do the most good; Consequently,

for best results cleaningY air-shouldv be suppliedY to the ring on the downstroke only.

Where the ring is reciprocated rapidly so that there is little difference in. cleanliness of one section of the lter over another, this precaution is of much less importance.

The apparatus in the preferred form forl carrying out the method is shown in the accompanying drawings, in which:

Figure 1 is a View in elevation of the improved dust filter;

Figure 2is a` plan of the same;

Figure 3 is a Vertical sectionon the line 3--3I of Figure 1;

Figure 4 is a perspective view of the` cleaning rmg;

Figure 5 is a diagrammatic view showing switch control;

Figure 6 is a sectional detail showing the relation of the cleaning ring and ilter bag;

Figure '7 isa sectional detail illustrating, the relationshipbetween the tension and. the, compression zone;

Figure 8 is a similar view illustrating the approximate relationshipbetween the forces operating on the filter fabric adjacent the cleaning jet;

Figure 9 is a sectional detail illustrating the action of the cleaning ringv and cleaning air.. on the downstrokeof the ring and li'igure 10 is a similar view for the upstroke of the ring.

The invention, in a Ipreferred illustrative form, includes a hopper lll, having a cylindrical wall II, a conical collector I2, and a `discharge chute I3 provided with a cut-off I4, manually operable through a handle I5.

A relatively low `pressure blower I6, driven by a motor Il, has an air intake I8 for the dust-laden air, and an outlet I9 connected to the lower terminal of air pipe 23, the end oi the pipe extending through the hopper wall II connected to the outlet I5 by a removable pipe section 2i. A platform 28 is suspended from a suitable fixture 32 by hangers 33, and in turn supports the tcp closure 2l of a filter bag '25. The lower end of the lter bag is connected to the upper end of the cylindrical wall t I of the hopper, and suchv filter is disposed in concentric spaced relation with the air pipe 23. The upper end of the air 4pipe 23 terminates hele-w the upper end of the filter bag 26 as an open preferably flaring end 24, so that the dust-laden air delivered by the blower will be directed through pipe 23, and delivered to the lter bag near the upper end of the latter and forced downwardly under blower pressure toward the hopper. The air cleared of impurities will pass through the filter bag and the dust particles separated therefrom will fall by gravity and under `blower pressure into the hopper.

The filter bag is kept clean by a hollow annular cleaning ring 5I, Figure 4, of split form with terminal ears 54 connected together by a bolt or the like 55. The cleaning ring 5I is formed with a narrow slit 5l through its inner peripheral margin and has an air inlet connection in the form of a nipple 53, having a valve 58a controlled by a lever 58h.

rIhe cleaning ring is designed to be in contact with the outer surface of the i'llter bag 26, when the latter is distended, the internal diameter of the ring being reduced by tightening the connector 55 until it is somewhat less than the external diameter of the distended filter bag 25. The ring will thus act to press the lter bag slightly inward throughout an area commensurate with the thickness of the ring, and of which area the narrow slot 5l forms the substantial center. The space between the terminal ears allows the bag to expand at that point and thereby compensate for the ycontraction of the bag by the ring over the remainder of the circumference of the bag. The high pressure air admitted will be discharged through the lter bag, its escape in any other direction being prevented by the contact of the ring and bag.

Supported on the platform 28 is an air compressor 3d, driven by a motor 35 by belt 36, the motor also driving a transverse shaft 39, mounted below the platform and having a length exceeding that of the diameter of the filter bag 26. The driving connections between the motor an-d the shaft 3S includes a speed reduction unit 38, driven through belt 3l by motor 35 and gears 4B. The loutlet of the air compressor 3d is connected by a iiexible hose 59 with the air inlet nipple 58 of the cleaning ring 5I. Air is thus continuously directed through the slot 5l of the ring 5I and through that portion of the filter bag 26 with which the slot may be in register so long as the compressor is operating.

Provision is made for reciprocating the cleaning ring 5I vertically to and from throughout substantially the entire length of the filter bag 25. In providing for this movement of the cleaning ring the ends of shaft 39 on the platform are proie vided with sprocket wheels 42 connected by chains 5 to corresponding sprocket wheels 43, mounted on stub shafts 44, carried by bearings 45, the 5 wheels 43 being retained on the shafts 44 by collars 45. .The Ibearings 45 are adjustably supported by a ring 48 by threaded rods 47. By turning the rods 4l the vertical position of the lower sprockets 43 can be varied as desired.

The sprockets 42 and 43 are thus arranged in aligned pairs in diametrically-opposed relation with and :beyond the filter bag 2li, as clearly shown in Figure 3. A connector, preferably a chain length 6| attached to each chain by a pin 62 and further connected to appropriate diametric pins extending outwardly from the cleaning ring 5i. The chain lengths 6I are longer than the diameter of the sprockets 42 so that when the pins E2 pass over the tops of the sprockets the pins Si! are below the bottoms 0f the sprockets 42. Consequently the ring 5I is raised to a position slightly Ibelow the sprockets 42 and then lowered after the pins S2 start their downward movement. The friction of the ring 5I on the distended felt cylinder is sufficient to support the ring against the force of gravity. Hence the Idownward movement of the ring does not commence until the chain lengths have been stretched out downwardly to =pull the ring down. Consequently the ring will not move down so far that its pins El) interfere with the lower sprockets 43.

For reasons stated above it is desirable that the cleaning ring exerts its cleaning function only on its downward movement as then the cleaned surface of the bag is above the cleaning ring. To accomplish this means must lbe sprovided to admit air under pressure to the cleaning ring in its downward movement and prevent such air admis-sion during upward travel of the ring.

To provide for this control of the air admission to the cleaning ring, a trip 5l is provided at the upper end of the lter bag 26 in position to be engaged by the valve controlled lever 53h of the air inlet to the cleaning ring, and in this engagement operate the lever to open the valve and admit air under pressure from the blower 313 to the ring. At its lower limit of movement, the cleaning ring valve controlled lever 53h engages a lower trip 68 in line with the upper end of the cylindrical body II of the hopper I0. In this engagement, the valve 58 of the cleaning ring is closed, so that during the upward movement of the ring, no air is admitted thereto, and no cleaning of the filter bag occurs.

As all the cleaning air has to pass outwardly through the i'llter it is desirable, not only that the use of such air be restricted to the down stroke of the cleaning ring but also to such periods as the filter requires cleaning. For this purpose the operation of the motor 35 is controlled by the pressure within the bag. When the pressure exceeds a predetermined figure the motor starts. When it falls below a predeter- 'e's mined figure the motor stops. Any conventional or desired type of pressure switch may be used so that description of such a switch is unnecessary and it is indicated symbolically only on the drawings at 63.

Thus, the'cleaning ring is brought into operation only when the lter bag reaches a predetermined constructed condition, and functions as a cleaning agent only on its down stroke over the fllterbag.

In operation,

therair or other medium to be agentes filtered is fed :by the blower .I6 to .and through the vtube 23 .and :delivered 'under lpressure tothe interior of the lter .bag near ithetop. Thema- 4terial received yby the filter bag is in the most part vdelivered under Vpressure to Athe hopper lil, but the liner particles collect in the surface of the Afilter bag and tend to reduce the filtering efficiency. This tends `to retard the;free ow of the air through .the filter., and Whena predetermined pressure condition is builtup, .the switch 63 I'is closed, .the motor 35 energized, and the chains 50 .operated to reciprocate the :cleaning ring `5l. As 'the vring vmoves :down .the exterior surface .of the filter bag, air under pressure is delivered vby .the ring to 'and through the filter, clearing the inner surface .of thefine :embedded particles. When the ring reaches the lower end of .the lter bag, the airflow'therethrough is cut off by the trip 68, and on the upward movement ofthe ring, no air is forced through the filter bag. At the vlimit ofthe `.upstrol:e, the trip 67 opens 'the valve 58 of the'ring, 'and thefairunder pressure is Yadmitted to the ring for clearing 'the lter bag.

One of the f-most important features o'f .the'inventionis the operative relationship between the cleaning ring-and lterbag. Asshown in Figure 6, it will be .noted that'as'th'e internal diameter of -the'ring is vless than the external diameter oi the bag v.when distended, the ring produces a concavo-convex portion in the bag Where the ring is .in contact therewith.

This has two important results. First,thein ternal pressure lon 'the bag Yproduces forces -along the lines AB-AB Vwhich pull the 'bag outwardly against the inner periphery of the cleaning ring. The resultant of these forces is indicated by AC. The cleaning air produces a force which is represented as AD.` 'So long as ACexceeds vADthere will be no lateral leakage of cleaning air.

Secondly, the bending v`of Athe Vfilter bag into concavo-.convex form tends to -open up the interstices on :the inner wall of the lter. Examination of felt Vfilters which have been -in'use for several months showed that 'practically vnone of the dust penetrated more than `halfwaythrough the felt. As shown diagrammatically in Figure 8, the felt has two zones, atension and linterstice expansion Zone and a compression Izon'e. 'The exact location 'of the line dividing rthetwozones is not Iimportant especially 'as in 'all probability due :to the forces AB-AB, the 'interstce 'expansionzone Vextends further into -the felt than any appreciable 'quantity of dust'has-'entered As previously explained, vthis spreading or opening of the bagthroughout l"this area on the innersicle tendsto more easily'dislodge the finer particles and to make more -'effective the blast from 'the cleaning ring. In this connection, yit will benoted that the outer sideof the filter bag is notsimilarly spread and hence the normalinterstices of the fiberfof the bag remain in 'their normal condition.

Figures 9'and10 indicate diagrammatically the difference in the flow of :lair inside-theflter-when the cleaningring'ismoving downwardly compared with the flow 'of air .when itiisfmovingiupwardly.

What 1 claim is.:

l. A dust lter comprising a cylindrical filter clement having its axis `.vertically ,disposed :and formed .of a exible sheet materialhaving .interstices much'smaller than .its thickness, .ineans'for supplying dust laden gases .to lthe upper interior portion of said element at a pressure greater than that 'on .the outer .surface thereof, a :horizontally disposed tubular member pressed inwardly against `tlreouter surfacev of said element -to cause the latter .to bend around said member-.underthe lpressureof the Vdust laden gases and thereby elongate the interstioes '.of said elementen the inner side thereof, said tubular member having 'a tslot therein throughits side contacting said element, means `for applying Arelatively high pressure 'gas 4to .said :tubular member :for discharge through saidslot, means for moving said tubular member to .and fro vertically whereby to .traverse thenouter side'of said element, :andmeans Afor interrupting 'the supply .of .high lpressure gasto said .tubular member :during Athe .up-stroke of said tubular member.

2. A- dust lter as in claiml whichalso includes means controlling isaid .moving means and 4high pressure gassupplying means for interrupting the movementof .'said tubular member and the'supp'lying of high pressure gas thereto whenever the pressure `drop through vthe filter element 'falls below a 'predetermined degree, and .resuming 'the movement yof said ftubularmember and the supply oi' high Lpressure gas thereto whenever `the pressure dropthrough'the filter element exceeds a :predetermined degree.

i3. 1A dust filter comprisinga cylindrical filter `element having its axis vertically disposed and formed of a flexible 'sheet material having'intersti-ces much smaller than-itsthickness, means for supplyingdust ladengases to the upper rinterior portion of said-element ata pressure greater'than that on .the outer surface thereof, a horizontally disposed tubular member pressed inwardly .gainst'theouter surface o'f said element to cause the-latter tobend aroundsaid member under the pressure oi' the dust laden gases and thereby elongate the interstioes of said element on the inner side Ythereof, said tubular member havinga slot therein through its side contacting said element, :means Vfor applying relatively high pressure 'gas .to said tubular member for discharge through said s'lot, means for moving said tubular member .to and Yfro vertically wherebyto traverse the outer side of saidelement, and means controlling'said moving means and high pressure gas supplying lmeans kfor interrupting the movement of said tubular member and the supplying of 'high pressure gas'theretowhenever'the pressure drop through the filter element falls'be'low a predetermined degree, and resuming the movement of said tubular member and the-supply of high pressure gas thereto whenever the pressure Idrop throughthelter element exceeds a predetermined degree.

HENRY J. HERSEY, Jn.

Vf'R/'EFRJENCES CITED The A.following references are of record in the file of this patent:

,UNITED {STA-TES PA'IENTS Number Name Date `996,860 .Kestner July .4, 1911 1,321,490 'Sweetland Nov. l'l, 1919 1,493,789 MullenvMay 13, 1924 FOREIGN .PATENTS Number yCountry Date 114,252 Australia Nov. 19, 1940 709,109 Germany July 3, 1941 '737,109 VGermany July '7, 1943 @erticate of Correction Patent No.. 2,495,635 January 24, 1950 HENRY J. HERSEY, JR.

Y It is hereby eertied that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 52, for the Word sufficient read insuicz'ent;

und that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and seeled this 30th dey of Mey, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

